AOSP not booting after OTA update (LoadImageAndAuth Failed)

I'm trying to update my custom AOSP ROM using OTA Full update. Despite I see no errors when sideloading (adb sideload ota.zip) or installing using UpdateEngine the OTA package, bootloader fails to load the updated ROM.

LoadImageAndAuth Failed: Load Error

RecoverySystem.verifyPackage() finds no errors.

To build OTA packages I use the following cmds:

lunch aosp_walleye-userdebug
make dist DIST_DIR=dist_output

It is odd that the compiled OTA package is named aosp_walleye-ota-eng.andrey.zip. ENG instead of userdebug.

The ROM itself can be installed and loaded by the device with no problems.

If I compile eng build, OTA works well (bootloader boots it with no errors).

lunch aosp_walleye-eng
make dist DIST_DIR=dist_output 

Are there any ways to debug the error?

My device is Pixel2.

The problem was with slots. Slot A had my custom ROM. Slot B did not.

It seems that both slots had to be flashed beforehand with the custom ROM to be able to boot full OTA update.